Copyright © Karl Dahlke, 2023
From 1893 to 1936, floating air ships travelled across the Atlantic, carrying passengers between Europe and America in style. They also flew across Siberia, and other barren or mountainous regions that were otherwise difficult to navigate. Recall the novel, Around the World in 80 Days, published in 1873 by Jules Verne. The first leg of the journey employed a balloon to cross the Alps. Ten years prior, in 1863, he published Five Weeks in a Balloon. Clearly Jules was fascinated with this technology, decades before it became practical.
These ships were held aloft by hydrogen, which is lighter than air. In fact it is the lightest substance known. Economics pushed manufacturers to use hydrogen, because you can lift more payload for the same volume of gas. It is also easy to produce; just split water into hydrogen and oxygen. Before we take a trip on one of these air ships, let's calculate the lifting power of hydrogen.
Recall that molecules in a gas have a special property. For a fixed volume, like a liter, or a gallon, at a fixed pressure, and a fixed temperature, there are the same number of molecules, no matter what the gas is. We can't say that about liquids or solids. We don't know if the molecules are tightly packed, or loosely packed. But in a gas, the molecules fly about, according to temperature and pressure, and there is always a fixed number of them in that container. If the molecules themselves are heavier, the gas is heavier. If the molecules are lighter, the gas is lighter. It's that simple.
How heavy is a molecule? That is determined by the atomic weight of the atoms in the molecule. Consider nitrogen, N2. Nitrogen has atomic weight 14, and there are two of them stuck together by a triple bond, thus nitrogen gas has weight 28. This is most of our atmosphere, most of the air we breathe. In fact, N2 is 80% of our air, and O2 is 20%. Oxygen gas has weight 16 times 2 = 32. Average them out, and toss in just a bit of other trace gases, and air has a weight of 29. Any gas that is heavier than 29 is heavier than air, and can't lift a thing. Consider argon, symbol Ar. It's atomic weight is 40. This is a nice safe gas, and we could pull it out of the atmosphere and put it in a balloon, but it is heavier than 29, and thus it has no lifting power.
Let's catalog all the gases that are lighter than air. There aren't very many of them. If the gas contains even one atom beyond silicon, it is too heavy. Even silicon, weight 28, is practically the same as air. It wouldn't even lift the balloon, much less the air ship beneath. Just for the curious, it can be made into a gas, silane gas, SiH4, with some interesting properties, but that is beyond the scope of this book. Sodium, magnesium, and aluminum are all metals that do not form gases. We can restrict our search to the first ten elements, hydrogen through neon. Here are the gases, in ascending order.
Any gas beyond ammonia is impractical. When combined with the chambers to contain it, it wouldn't lift the payload.
Borane looks promising, but it's a trick. A room full of borane quickly self-combines to make diborane. Just as a room full of individual nitrogen atoms, if such could ever be created, would quickly self-combine into nitrogen molecules, N2. So let's cross borane off the list. That leaves hydrogen, helium, methane, and ammonia.
Unfortunately hydrogen is extremely flammable. Hydrogen + oxygen is the fuel of the space shuttle, and other NASA rockets. This was well understood from the outset, and it severely restricted the comfort and convenience of the passengers. The only open flame on board was a small cook stove in the galley. Other than that, there was no heat - and it gets plenty cold 2,000 meters up, with temperatures hovering just above freezing. Passengers spent a week crossing the ocean encased in winter clothing and wrapped in blankets. It was a miserable experience. In contrast, crossing by ship also took a week, in a heated stateroom, for the same price. But the Zeppelin corporation persisted.
There was another inescapable source of flame - the engines. A ship could hardly be at the mercy of the winds, that was not practical. Propeller engines, similar to those on the first airplanes, were placed along the sides of the air ship, well away from the hydrogen balloons. These moved the ship forward, at a speed of 19 knots, and later, 40 knots, with improved design. If there was a favorable tail wind, the ship could travel at almost 60 knots.
The engines could run on gasoline, like the airplane engines of the day, but that represented extra weight that had to be carried aloft. Instead, they ran on blau gas, named for Hermann Blau, which was literally as light as air. This gas was stored in large chambers, just like the hydrogen. It consisted of a mix of hydrocarbons, methane, ethane, etc, having an average weight of air. Thus the fuel, needed to propel the ship on its voyage, did not weigh it down.
Every engine produces waste heat, and eventually, they learned how to redirect this heat back into the cabin. Passengers could finally travel in comfort, without freezing to death. With increased comfort and speed, air ships became more popular.
There was another need, a desperate need, of passengers in the early 20th century. Almost everyone was addicted to cigarettes. They couldn't imagine going a week without a smoke. You've seen the movie Apollo 13 - everyone in Mission Control was smoking. Ron Howard got that right. But what about that huge tower of hydrogen overhead? What if there was a leak in one of the chambers? Hydrogen has no color, and no odor. The leak would go unnoticed, until a tongue of fire ran from the end of your cigarette up to the chamber, followed by an explosion in the sky. To avoid this, Zeppelin built a closed smoking room within the ship. Matches or open flames were not permitted. The room contained an electric cigarette lighter, that was affixed to the wall and could not be removed. Several times a day, passengers would enter this room and get their nicotine fix. They were riding in a floating bomb, but they still had to smoke.
In 1936, the Hindenburg burst into flames as it tried to land in New Jersey, the terminus of its trans-Atlantic flight. The circumstances surrounding this disaster are not well understood, even today. However, one thing is certain - with pictures of the burning ship in hand, passengers were not willing to fly by hydrogen any more. air ships turned to helium, which is an inert gas. It doesn't combine with anything, and is perfectly safe. Furthermore, it has almost the same lifting power as hydrogen. However, it is much more expensive. Helium is the quintessential non-renewable resource. It is already scarce, and being an element, it can never be created. It must be mined from natural gas reserves at a substantial cost, and once that source is exhausted, there is no more helium.
The easiest way to descend in a blimp is to vent the helium, but we don't want to do that. When helium enters the atmosphere it escapes out into space, too light for earth's gravity to hold. It is forever gone. We need to recapture the helium - pull it out of the chambers and put it back into a canister for future use. That increases the mechanical complexity of the craft.
Beyond blimps, industrial demand for helium includes particle accelerators, MRI machines, and even party balloons, which drives the price of helium up. MRI machines HAVE to have helium, no alternative that we know of, but what about our blimps?
Methane is almost as flammable as hydrogen - not a good idea. That leaves only ammonia. It's flashpoint is 93 °C, almost the boiling point of water. If there is a leak, and the ammonia streams across a lit cigarette, there is still no fire. Of course there is a pungent smell, and perhaps lung and eye damage if concentrations rise high enough, but we'll assume there isn't a leak at that level. You're in the open air, which helps, hold your breath for 30 seconds and it will probably dissipate. Scientists have proved the concept - ammonia balloons have carried passengers and payloads aloft for test flights.
Let's do some math. Helium has weight 4, thus a lifting power of 29 - 4 = 25. Ammonia has weight 17, thus a lifting power of 29 - 17 = 12. 12 is a bit less than half of 25, and that's a bummer. You need twice the volume, twice as many chambers, to carry the same weight. That is an engineering challenge to be sure, but then again, we make millions of tons of ammonia every year, so the gas is cheap. And no worries about venting it in the atmosphere to descent, just fill up again when you're on the ground. I think the day will come, eventually, when the curves cross, and ammonia blimps become more economical than helium.
No matter the lifting gas, blimps are not practical for trans-Atlantic flights any more, as airplanes have completely usurped this market. Nobody wants to spend 4 days crossing the ocean, when 7 hours would suffice. How about shorter flights?
If all goes well, blimps will soon be used for air travel again in Europe, for short trips. They sport small engines, soon to be electric, for lateral propulsion. the lifting gas is helium. It is still more economical than ammonia, for now. These are large craft, larger than a Boeing 747. They are expected to produce up to 90% fewer emissions. At 130 kph, they are not particularly fast, but still fast enough for short hops.
What is the heaviest gas that can (theoretically) exist? The answer might be uranium hexafluoride, UF6. This gas was produced in the 1940's, as part of the uranium enrichment project at Oakridge Tennessee. Enriched uranium, with a higher concentration of isotope U235, acts as the fuel for our atomic bombs and our nuclear reactors. Therefore, this gas can definitely be produced in substantial quantities.
If a heavy molecule is to remain a gas, it must be compact and symmetric. Indeed that is the case. Each uranium atom is surrounded by six fluorine atoms, up down left right front and back. These little molecules bounce around and do not stick together, despite their relatively large mass. With a molecular weight of 346, this gas is nearly 12 times as heavy as air.
I'm cheating a bit here, because it isn't a gas at room temperature. It is a white solid. When heated to 56 degrees C, or 133 degrees F, a modest temperature, it sublimes into a gas. As either a solid or a gas, it reacts violently with water to produce hydrofluoric acid. It is toxic, and brief exposure can cause death or injury.
There is a pattern here. Many elements can be combined with six fluorine atoms to produce a heavy gas. The first is sulfur hexafluoride, SF6. It boils at -51 degrees C, so it is certainly a gas at room temperature. With a molecular weight of 140, it is nearly 5 times as heavy as air. It is also safe to breathe, i.e. nontoxic. Since it is heavier than air, it changes the resonance of the voice to a lower register. The more familiar helium is lighter than air, and it changes the voice resonance to a higher register.